The present invention relates to a control rod for a boiling water reactor comprising four absorber blades forming an orthogonal cross. The absorber blades contain absorber material distributed in the longitudinal direction, a mean value of the quantity of absorber material per unit of length of the control rod being smaller in the upper part of the control rod than in the lower part thereof.
A core in a boiling water reactor normally comprises several hundred fuel assemblies. These are arranged vertically in the core and have an at least substantially square cross section. Each fuel assembly comprises a bundle of fuel rods. In boiling water reactors, fuel bundles containing 8xc3x978, 9xc3x979 or 10xc3x9710 fuel rods are often used. A fuel bundle is surrounded by a fuel channel. The fuel channel is open at both ends so as to allow the coolant of the reactor to flow through the fuel bundle. The core is immersed into water which serves both as coolant and as neutron moderator. Each fuel rod contains a large number of fuel pellets stacked on top of each other in a cladding tube.
A nuclear reactor also comprises a plurality of control rods, the primary task of which is to start, control and shut off the power generation by being inserted into and extracted from the reactor core. In the boiling water reactor, the control rods are inserted into the core from below. With the aid of a drive, the control rod may be operated in different positions between a fully extracted and a fully inserted position in the core. Certain of the control rods are extracted from the core during operation, whereas others are inserted to different levels. These levels vary during the operating cycle of the reactor. In the boiling water reactor, the control rods are composed of four vertically arranged elongated absorber blades, which together form an orthogonal cross. The absorber blades are generally of stainless steel and provided with a large number of bored holes or tubes filled with a neutron-absorbing material, for example boron carbide (B4C) or hafnium. The absorber material is usually uniformly distributed in both the axial and radial directions in the absorber blades. The active length of the control rods, that is, the length of that part of the control rod which contains the absorber material, corresponds to the active height of the reactor core when the control rods are fully inserted into the core.
The fuel assemblies are arranged in a regular lattice, where each fuel assembly is included in two rows of fuel assemblies which are perpendicular to each other. The control rods are normally arranged with each one of their absorber blades between two fuel assemblies located in the same row, such that each control rod together with four fuel assemblies arranged around its blades form one unit.
The control rods in the reactor are usually divided into two groups with different tasks. One group of control rods is, during normal operation, fully extracted from the core and used only for stopping and starting the reactor. The other group is used for controlling the reactor power and for burnup compensation during normal reactor operation. The control rods in the second group will hereinafter be referred to as controlling rods. These controlling rods normally constitute less than 10% of all the control rods. At the beginning of an operating cycle, the controlling rods are inserted. By an operating cycle is meant the time between two refuellings. To compensate for the burnup of the fuel, the controlling rods are successively extracted during the operating cycle. The change of the positions of the controlling rods takes place at certain time intervals, for example once a week. The change may, for example, consist of the controlling rods being extracted about 8% of their lengths.
Currently, conventional control rods are used, with the absorber material uniformly distributed in the absorber blades, in both groups of control rods. During the time when the controlling rods are inserted, those fuel pellets which are located adjacent to the absorber blades are protected from burnup of fissile material. This leads to a situation where, each time the controlling rod is extracted a certain distance, fuel pellets containing high contents of fissile material are exposed, resulting in a considerable increase in power in these fuel pellets. This local increase in power takes place very rapidly. Such stresses may in certain cases cause fuel failure, so-called PCI failure (PCI=pellet-clad interaction), on the cladding tube surrounding the fuel pellets. The fuel rods which are located nearest the cruciform centre of the controlling rod are particularly subjected to such stresses and hence run the greatest risk of fuel failure. The reason for this is that these fuel rods are protected against burnup of fissile material by two absorber blades.
According to one previous solution, a control rod is known which in its upper part if arranged with a smaller percentage of absorber material per unit of length than the rest of the control rod. In this way, part of the fissile material is burnt up during the time when the control rod is fully inserted into the core and the power increase is not equally great when the controlling rod is extracted. A reduced power increase when the controlling rod is extracted is achieved by arranging a larger number of bored channels perpendicular to the longitudinal direction of the control rod and filling them with a neutron-absorbing material, whereby the channels in the upper part of the control rod have a relatively smaller radius, whereby the length of the respective channel is considerably larger than its radius, whereas the other channels have a relatively larger radius.
One disadvantage of the above-mentioned control rod is that the reduction of the power increase obtained when the control rod is extracted is not sufficiently great to eliminate the risk of fuel failure on the most exposed fuel rods, that is the fuel rods situated nearest the cruciform centre of the control rod.
Another disadvantage is that the service life becomes shorter for the above-mentioned control rod than for conventional control rods. The service life of the absorber material, and hence the service life of the control rod, depend on the quantity of absorber material per unit of surface. For a cylindrical channel, the length of which is considerably larger than its radius, the service life of the absorber material is substantially influenced by its radius. A reduction of the diameters of the channels leads to a corresponding reduction of the service life of the control rod.
According to another previous solution, it is known to arrange a smaller quantity of absorber material in that part of the absorber blades which extends along the fuel rods located nearest to the cruciform center. In their outer part, the blades are provided with a larger number of channels, filled with absorber material, which extend perpendicular to the longitudinal direction of the control rod. In one embodiment, that part of the absorber blades which is located inside the filled channels lacks absorber material. The control rod is provided with recesses, which extend across the fuel rod located nearest the cruciform center. These recesses are filled with moderator such that the consumption of fissile material in adjacent fuel rods increases. By arranging the absorber material in this way, it is possible to considerably extend the duration of an operating cycle for a reactor.
The disadvantage of using such a control rod as a controlling rod is that the risk of fuel failure admittedly has been reduced for the most exposed fuel rods, that is, those located nearest the cruciform centre, but the risk of fuel failure on the other fuel rods is unchanged compared with the risk of fuel failure when using conventional control rods.
The object of the invention is to achieve a control rod for a nuclear reactor which provides a reduced risk of fuel failure when, from an inserted position for a longer period of time, it is extracted from the reactor core in successive steps.
What characterizes a control rod according to the invention will become clear from the appended claims.
A control rod according to the invention has absorber blades which form an orthogonal cross with a central cruciform centre, the capacity of which to absorb neutrons varies both axially and radially. The capacity of the absorber blades to absorb neutrons is lower in their upper part than in their lower part. In the upper part of the absorber blades, the neutron absorption capacity is higher in its outer part than in its inner part which adjoins the cruciform centre. The control rod according to the invention leads to the power reduction for fuel pellets located adjacent to the upper part of the control rod becoming smaller in relation to fuel pellets which are not controlled with respect to fission velocity with the aid of control rods. Especially those fuel pellets which are located nearest the cruciform centre of the control rod will have a lower power reduction than those fuel pellets which are not controlled with the aid of control rods. When the control rod is extracted somewhat, and the fuel pellets which have been protected by the upper part of the control rod are exposed, the power increase in these fuel pellets will be smaller than for prior art control rods, which is due to the fuel pellets already being burnt up to a certain extent.
One advantage of a control rod according to the invention is that the risk of fuel failure on the cladding tube in connection with the control rod being extracted is reduced. Another advantage is that the utilization of the fuel becomes more efficient. An additional advantage is that longer service lives of the control rods are made possible.
In a control rod according to the invention, each one of the absorber blades comprises an upper and a lower part, the mean value of the contents of absorber material per unit of length of the control rod being smaller in the upper part of the control rod than in the lower part thereof. The upper part comprises an outer part provided with absorber material and an inner part which lacks absorber material and which is arranged radiallly inside the outer part.
To achieve the advantages of the invention, at least some portion of the above-mentioned inner part should constitute at least one-fourth of the width of the absorber blade in the radial direction. In an especially advantageous embodiment, at least some portion of the above-mentioned inner part should constitute at least one-third of the width of the absorber blade in the radial direction. The area, in a section across the longitudinal direction of the control rod, of the above-mentioned inner part should be at least 25% of the area of the upper part. In an especially advantageous embodiment, the area, in a section across the longitudinal direction of the control rod, of the above-mentioned inner part should be at least 30% of the area of the upper part. The length of the upper part should not exceed one-third of the total length of the absorber blade.
In a preferred embodiment of the invention, recesses in the form of through-holes are arranged in the inner central parts of the upper part of the control rod. This implies that more neutron moderator is supplied to the upper central part of the control rod, which further increases the burnup of fissile material in the fuel rods which are arranged nearest the cruciform centre of the control rod.